1. Field of the Invention
The present invention relates to a manufacturing method of a zeolite structure. More particularly, it relates to a manufacturing method of a zeolite structure having an excellent mechanical strength.
2. Description of the Related Art
It is known that zeolite is a type of silicate having a network crystal structure provided with fine pores having a uniform diameter, there are present various chemical compositions represented by the general formula: WmZnO2n.sH2O (W: sodium, potassium, calcium or the like, Z: silicon, aluminum or the like, and s takes various values), and there are present many kinds (types) of crystal structures having different pore shapes. Zeolite has an inherent adsorption ability, catalyst performance, solid acid characteristics, ion exchange ability and the like based on the respective chemical compositions or crystal structures, and is utilized in various use applications such as an adsorbing material, a catalyst, a catalyst carrier, a gas separation film, and an ion exchanger.
For example, MFI-type zeolite (also referred to as “ZSM-5 type zeolite”) is zeolite provided with pores each having a size of about 0.5 nm by oxygen ten-membered rings among crystals, and is utilized in a use application such as an adsorbing material which adsorbs nitrogen oxides (NOx), hydrocarbons (HC) or the like from a car exhaust gas, or a gas separation film which selectively separates only p-xylene from a xylene isomer. Moreover, Deca-Dodecasil 3R (DDR) type zeolite is zeolite provided with pores of about 0.44×0.36 nm by oxygen eight-membered rings among crystals, and is utilized in a use application such as a gas separation film which selectively separates/removes only carbon dioxide from a natural gas or a biological gas to improve purity of methane which is useful as a fuel.
Moreover, for the purpose of removing NOx or the like contained in an exhaust gas discharged from an engine for a car, an engine for a construction machine, an industrial stational engine, a burning apparatus or the like or adsorbing hydrocarbons or the like from the exhaust gas, as a ceramic carrier (a honeycomb structure) made of cordierite or the like and having a honeycomb shape, there is used a catalyst material onto which zeolite subjected to an ion exchange treatment is loaded.
When zeolite is loaded onto the above ceramic carrier made of cordierite or the like as described above, cordierite or the like does not exert a function of removing NOx, a function of adsorbing hydrocarbons or the like. Therefore, when cordierite or the like is present, a pressure loss during passing of the exhaust gas, accordingly, increases.
To solve this problem, there is suggested a method of forming and firing a forming raw material containing zeolite subjected to an ion exchange treatment between cations of zeolite and metal ions, to form a honeycomb structure itself (e.g., see Patent Documents 1 to 4).
[Patent Document 1] JP-A-2007-296521
[Patent Document 2] Japanese Patent No. 3272446
[Patent Document 3] JP-A-2008-169104
[Patent Document 4] WO2009/141878A1
However, the above conventional zeolite structure has a problem that a mechanical strength such as a bending strength is low. In particular, when the honeycomb structure itself is made of zeolite, the structure is installed and used in an exhaust system of a car. Therefore, the conventional zeolite structure has a problem that the structure is broken or deformed due to vibrations of the car or the like.
To solve the problem of the mechanical strength in the conventional zeolite structure, for example, Patent Document 3 discloses a zeolite structure including silica sol as an inorganic binder, to improve a mechanical strength. However, heretofore used typical silica sol has a spherical shape, is stabilized with sodium ions, and contains stable (alkaline) silica particles in an alkaline region. Such silica sol is mixed with zeolite particles having acidity and neutralized. When silica sol shifts to a neutral region, silica particles in silica sol are mutually agglomerated. Therefore, binding points between silica particles and the zeolite particles decrease, and silica particles cannot sufficiently be bound to the zeolite particles. The zeolite structure including heretofore used typical silica sol has a problem that a mechanical strength thereof lowers.